Finally, the writer notes that the decline in the amphibian population has been blamed on the introduction of trout into the park’s waters in 1920, but then dismisses that argument on the purely specious basis that it does not explain the worldwide decline. This part of the argument blithely dismisses the very relevant fact that trout are known to eat amphibian eggs. This attempt to "prove a negative" is the last resort of those in search of some vain attempt to prove the truth of the matter that they are asserting. It is basically impossible to "prove a negative"; this is an attempt to shift the burden of proof back on to the nonbelievers of the argument. The global environmental situation and that of Yosemite National Park are not perfectly correlated, and the fact that the trout may very well be responsible for the decline cannot simply be dismissed without further proof.
In summary, the writer fails to establish any causal relationship between global air and water pollution and the decline of amphibious life worldwide. The evidence presented is extremely weak at best and narrowly focuses on one tiny area of the globe, as well as putting forward as proof two studies about which almost nothing is known. For a stronger argument, the writer would need to directly put forth evidence associating air and water pollution with not only the decline at Yosemite but also throughout other areas of the world.